Combining two frequencies in close proximity is a difficult engineering challenge, especially when trying to maintain high efficiency, flat group delay, low input VSWR, and high transmitter to transmitter isolation. A typical two frequency FM multiplexer can combine two stations 800 kHz or more apart. The worst case would be 0.75% bandwidth separation combining 107.1 MHz and 107.9 MHz. Existing technology uses two 4-pole filters with a cross coupling from the one-to-four cavities. Efficient combining of closer adjacent frequencies is much more difficult.
High level IBOC combining methods attempt to implement a much narrower bandwidth separation of 0.04%. Prior to the present invention, this was accomplished either with a 10 dB hybrid coupler or with a filter solution using bandpass filters as described in Fanton, M, “IBOC combining schemes for 10 dB injection,” in NAB BEC Proceedings, 2009, pp. 122-123; and Small, D, “A Low-Loss IBOC Combining Method,” Radio World, 2010. http://www.rwonline.com/printarticle.aspx?articleid=2744.